diff --git a/backends/cxxrtl/cxxrtl.h b/backends/cxxrtl/cxxrtl.h
index d2f06d6bf..d4e2fa7fc 100644
--- a/backends/cxxrtl/cxxrtl.h
+++ b/backends/cxxrtl/cxxrtl.h
@@ -905,10 +905,7 @@ std::ostream &operator<<(std::ostream &os, const value_formatted<Bits> &vf)
 				buf += '0';
 			while (!val.is_zero()) {
 				value<Bits> quotient;
-				if (negative)
-					val.signedDivideWithRemainder(value<Bits>{10u}, quotient);
-				else
-					val.divideWithRemainder(value<Bits>{10u}, quotient);
+				val.divideWithRemainder(value<Bits>{10u}, quotient);
 				buf += '0' + val.template slice<3, 0>().val().template get<uint8_t>();
 				val = quotient;
 			}
diff --git a/kernel/fmt.cc b/kernel/fmt.cc
index 0fac64175..0fdd996fb 100644
--- a/kernel/fmt.cc
+++ b/kernel/fmt.cc
@@ -680,8 +680,8 @@ std::string Fmt::render() const
 							if (absvalue.is_fully_zero())
 								buf += '0';
 							while (!absvalue.is_fully_zero())	{
-								buf += '0' + RTLIL::const_mod(absvalue, 10, part.signed_, false, 4).as_int();
-								absvalue = RTLIL::const_div(absvalue, 10, part.signed_, false, absvalue.size());
+								buf += '0' + RTLIL::const_mod(absvalue, 10, false, false, 4).as_int();
+								absvalue = RTLIL::const_div(absvalue, 10, false, false, absvalue.size());
 							}
 							if (negative || part.plus)
 								buf += negative ? '-' : '+';
diff --git a/tests/fmt/fuzz/CMakeLists.txt b/tests/fmt/fuzz/CMakeLists.txt
index 6f7c43da8..e43606772 100644
--- a/tests/fmt/fuzz/CMakeLists.txt
+++ b/tests/fmt/fuzz/CMakeLists.txt
@@ -17,6 +17,7 @@ add_executable(
     x_test
     x_test.cc
     ../../../libs/bigint/BigUnsigned.cc
+    ../../../libs/bigint/BigInteger.cc
 )
 
 link_fuzztest(x_test)
diff --git a/tests/fmt/fuzz/x_test.cc b/tests/fmt/fuzz/x_test.cc
index ce4af5413..07ff7885d 100644
--- a/tests/fmt/fuzz/x_test.cc
+++ b/tests/fmt/fuzz/x_test.cc
@@ -4,34 +4,33 @@
 #include <sstream>
 #include "backends/cxxrtl/cxxrtl.h"
 #include "libs/bigint/BigUnsigned.hh"
+#include "libs/bigint/BigInteger.hh"
 
 using namespace cxxrtl_yosys;
 
-void Formats128BitUnsignedIntegers(uint64_t x, uint64_t y) {
-  // Compare output to actual BigUnsigned.
-  value<64> vs;
+void Formats128BitUnsignedIntegers(chunk_t x0, chunk_t x1, chunk_t x2, chunk_t x3)
+{
+  // Compare output to BigUnsigned.
   value<128> v;
-  vs.set(x);
-  v = v.blit<127, 64>(vs);
-  vs.set(y);
-  v = v.blit<63, 0>(vs);
+  v = v.blit<127, 64>(value<64>{x1, x0});
+  v = v.blit<63, 0>(value<64>{x3, x2});
 
   std::ostringstream oss;
   oss << value_formatted<128>(v, false, false, ' ', 0, 10, false, false, false);
   auto actual = oss.str();
 
-  BigUnsigned b(x);
-  b.bitShiftLeft(b, 64);
-  b.bitOr(b, y);
+  BigUnsigned u;
+  u.bitShiftLeft(v.slice<127, 64>().val().get<uint64_t>(), 64);
+  u.bitOr(u, v.slice<63, 0>().val().get<uint64_t>());
 
   std::string expected;
 
-  if (b.isZero()) {
+  if (u.isZero()) {
     expected = "0";
   } else {
-    while (!b.isZero()) {
-      expected += '0' + (b % 10).toInt();
-      b /= 10;
+    while (!u.isZero()) {
+      expected += '0' + (u % 10).toInt();
+      u /= 10;
     }
     std::reverse(expected.begin(), expected.end());
   }
@@ -40,3 +39,41 @@ void Formats128BitUnsignedIntegers(uint64_t x, uint64_t y) {
 }
 FUZZ_TEST(CxxrtlDivisionFuzz, Formats128BitUnsignedIntegers);
 
+
+void Formats128BitSignedIntegers(chunk_t x0, chunk_t x1, chunk_t x2, chunk_t x3) {
+  // Compare output to BigInteger.
+  value<128> v;
+  v = v.blit<127, 64>(value<64>{x1, x0});
+  v = v.blit<63, 0>(value<64>{x3, x2});
+
+  std::ostringstream oss;
+  oss << value_formatted<128>(v, false, false, ' ', 0, 10, true, false, false);
+  auto actual = oss.str();
+
+  BigUnsigned u;
+  bool negative = v.is_neg();
+  if (negative)
+    v = v.neg();
+  u.bitShiftLeft(v.slice<127, 64>().val().get<uint64_t>(), 64);
+  u.bitOr(u, v.slice<63, 0>().val().get<uint64_t>());
+
+  std::string expected;
+  if (u.isZero()) {
+    expected = "0";
+  } else {
+    // Note that we never actually do division of negative numbers: our division
+    // routines are flooring, not truncating, so dividing by 10 repeatedly won't
+    // necessarily ever get to zero.
+    while (!u.isZero()) {
+      expected += '0' + (u % 10).toInt();
+      u /= 10;
+    }
+    if (negative) {
+      expected += '-';
+    }
+    std::reverse(expected.begin(), expected.end());
+  }
+
+  EXPECT_EQ(actual, expected);
+}
+FUZZ_TEST(CxxrtlDivisionFuzz, Formats128BitSignedIntegers);